Many different types of printing have been invented, a large number of which are presently in use. The known forms of print have a variety of methods for marking the print media with a relevant marking media. Commonly used forms of printing include offset printing, laser printing and copying devices, dot matrix type impact printers, thermal paper printers, film recorders, thermal wax printers, dye sublimation printers and ink jet printers both of the drop on demand and continuous flow type. Each type of printer has its own advantages and problems when considering cost, speed, quality, reliability, simplicity of construction and operation etc.
In recent years, the field of ink jet printing, wherein each individual pixel of ink is derived from one or more ink nozzles has become increasingly popular primarily due to its inexpensive and versatile nature.
Many different techniques on ink jet printing have been invented. For a survey of the field, reference is made to an article by J Moore, “Non-Impact Printing: Introduction and Historical Perspective”, Output Hard Copy Devices, Editors R Dubeck and S Sherr, pages 207-220 (1988).
Ink Jet printers themselves come in many different types. The utilization of a continuous stream of ink in ink jet printing appears to date back to at least 1929 wherein U.S. Pat. No. 1,941,001 by Hansell discloses a simple form of continuous stream electro-static ink jet printing.
U.S. Pat. No. 3,596,275 by Sweet also discloses a process of a continuous inkjet printing including the step wherein the ink jet stream is modulated by a high frequency electro-static field so as to cause drop separation. This technique is still utilized by several manufacturers including Elmjet and Scitex (see also U.S. Pat. No. 3,373,437 by Sweet et al)
Piezoelectric ink jet printers are also one form of commonly utilized ink jet printing device. Piezoelectric systems are disclosed by Kyser et. al. in U.S. Pat. No. 3,946,398 (1970) which utilizes a diaphragm mode of operation, by Zolten in U.S. Pat. No. 3,683,212 (1970) which discloses a squeeze mode of operation of a piezoelectric crystal, Stemme in U.S. Pat. No. 3,747,120 (1972) discloses a bend mode of piezoelectric operation, Howkins in U.S. Pat. No. 4,459,601 discloses a piezoelectric push mode actuation of the ink jet stream and Fischbeck in U.S. Pat. No. 4,584,590 which discloses a shear mode type of piezoelectric transducer element.
Recently, thermal ink jet printing has become an extremely popular form of ink jet printing. The ink jet printing techniques include those disclosed by Endo et al in GB 2007162 (1979) and Vaught et al in U.S. Pat. No. 4,490,728. Both the aforementioned references disclosed inkjet printing techniques that rely upon the activation of an electrothermal actuator which results in the creation of a bubble in a constricted space, such as a nozzle, which thereby causes the ejection of ink from an aperture connected to the confined space onto a relevant print media. Printing devices utilizing the electro-thermal actuator are manufactured by manufacturers such as Canon and Hewlett Packard.
As can be seen from the foregoing, many different types of printing technologies are available. Ideally, a printing technology should have a number of desirable attributes. These include inexpensive construction and operation, high speed operation, safe and continuous long term operation etc. Each technology may have its own advantages and disadvantages in the areas of cost, speed, quality, reliability, power usage, simplicity of construction operation, durability and consumables.
A common problem with inkjet printers is that an unavoidable number of ink droplets ejected from each nozzle are misdirected. By “misdirected”, it is meant that the ink droplet does not follow its intended trajectory towards a print medium. Usually, the intended trajectory of an ink droplet is perpendicular to an ink ejection surface of the printhead. However, some misdirected ink droplets may be ejected at a skewed angle for a variety of reasons.
In some cases, misdirected ink droplets may be a result of malformed nozzles or nozzle openings during the printhead manufacturing process. In these cases, the misdirected ink droplets will be systematic and generally unavoidable.
In other cases, misdirected ink droplets will be irregular and unpredictable. These may result from, for example, dust particles partially occluding nozzle openings, ink flooding across the surface of the printhead between adjacent nozzles, or variations in ink viscosity. Typically, an increase in ink viscosity will lead to a greater number of misdirects and ultimately result in nozzles becoming clogged—a phenomenon known in the art as “decap”.
Misdirected ink droplets are clearly problematic in the inkjet printing art. Misdirected ink droplets result in reduced print quality and need to be minimized as far as possible. They are especially problematic in the high-speed inkjet printers developed by the present Applicant. When printing onto a moving print medium at speeds of up to 60 pages per minute, the effects of misdirects are magnified compared with traditional inkjet printers.
Accordingly, a number of measures are normally taken to avoid the causes of misdirects. These measures may include, for example, low manufacturing tolerances to minimize malformed nozzles, printhead designs and surface materials which minimize ink flooding, filtered air flow across the printhead to minimize build up of dust particles, and fine temperature control in the nozzles to minimize variations in ink temperature and, hence, ink viscosity.
However, all of these measures significantly add to manufacturing costs and do not necessarily prevent misdirects. Even when such measures are implemented, some misdirects are inevitable and can still result in unacceptably low print quality.
It would be desirable to provide a printhead assembly, which gives improved print quality. It would further be desirable to provide a printhead assembly, which reduces the effects (in terms of reduced print quality) of misdirected ink droplets. It would still further be desirable to provide a printhead assembly, which gives robust protection of nozzle structures formed on the surface of the printhead.